This application claims priority to Chinese Application Serial Number 00136189.9 filed on Dec. 27, 2000, the disclosure of which is incorporated herein by reference in its entirety.
The present invention relates to a catalyst for the preparation of high molecular weight aliphatic polycarbonate and the preparation of the catalyst. The present invention further relates to a method for the preparation of high molecular weight aliphatic polycarbonate by using the catalyst of the present invention.
Venting of carbon dioxide to the atmosphere in large amount results in xe2x80x9cgreenhouse effectxe2x80x9d which is detrimental to the environment. Carbon dioxide is an environmental polluting gas, but from another point of view carbon dioxide is an abundant natural resource. One of the main hot spots for research is to utilize carbon dioxide as a raw material for synthesizing polymeric materials. These materials could be used as biodegradable packaging materials, medical materials, adhesives and additives for composites.
It was reported in U.S. Pat. No. 3,585,168, U.S. Pat. No. 3,900,424 and U.S. Pat. No. 3,9563,383 that alternating copolymers (Mn greater than 20,000) of carbon dioxide and epoxy compounds as well as various polyurethane and polyethers were synthesized by using alkyl zinc/compounds containing active hydrogen as the catalysts. In JP 02,575,199 and JP 02,142,824, expensive complex of metal and porphyrin was used as the catalyst. Although this catalyst gave higher catalytic efficiencies (103-104 g polymer/mol catalyst), the molecular weights of polymers obtained were relatively low (number average molecular weight (Mn) was approximately 5,000) and the polymerization time was more than 10 days. In Chinese Patent Application No. ZL89100701.6 and ZL91109459.8, anionic coordinated bimetallic catalytic systems were used. The catalytic efficiency was 103-104 g polymer/mol catalyst. Yet due to the difficulty of removing the supporting materials, polycarbonate thus obtained was hard to be purified. In addition, the molecular weight of the polymer thus obtained still needs to be raised. It was reported in Macromolecules, 24, 5305, 1991; Macromolecules, 30, 3147, 1997; and Polymer Preprint 99-3, 100, 2000 that a rare-earth ternary catalyst system can be used for the synthesis of polycarbonate. The former used rare-earth ester of phosphonic acid as the catalyst which has a lower catalytic efficiency and lower degree of carbon dioxide fixation ( less than 30 wt %). Polycarbonate obtained by this catalyst was a block copolymer. The latter paper described that the percentage of the alternative sequence structure was higher than 95%, but the coordination compound used was expensive and the catalytic efficiency needed to be raised. A comprehensive review on the catalysts and polymerization systems used in the syntheses of polycarbonate was published in J. Polymer Sci.: Part A: Polym. Chem. 37, 1863, 1999. It was pointed out that except for a few zinc salt catalytic systems (polymerization time was 40 hours), most of the processes were polymerization processes involving organic solvents like dioxane, dioxolane, dioxymethane, hexane, benzene, tetrahydrofuran, toluene or a mixture thereof. By using zinc salt of glutaric acid as the catalyst for non-solvent polymerization, it is possible to obtain polycarbonate with high catalytic efficiency. However it needs a polymerization time of 40 hours and also very critical polymerization conditions.
Chinese Patent Application No. 98125654.6 and 98125655.9 described methods for preparing rare-earth complex ternary catalyst and high molecular weight aliphatic polycarbonate.